1. Field of the Invention
The present invention relates to a pneumatic tire with ribs and/or blocks formed on shoulder portions of a tread, and in more detail, it relates to a pneumatic tire with inhibited uneven wear on said shoulder portions.
2. Description of the Prior Art
Uneven wear is likely to occur on shoulder portions of a tread of a pneumatic tire. As the uneven wear progresses, various performances of a tire are degraded, which requires exchange of tires in an early stage. Therefore, in order to inhibit uneven wear, various measures have been taken.
In a pneumatic tire of patent document 1 (Japanese Patent Laid-Open Publication No. Hei 6-87303), two sub-grooves extending in the circumferential direction are provided on both sides that sandwich a grounding end, thereby wearing the region sandwiched by said sub-grooves (wearing region) being sacrificed. Consequently, wearing does not progress further inside of the tire width direction from said region, thereby inhibiting the uneven wear of a tread. In addition, in the pneumatic tire of patent document 2 (Japanese Patent Laid-Open Publication No. Hei 5-262105), three fine grooves extending in the circumferential direction on the outer side of the grounding end including a grounding end are provided. In this case, too, with the sacrifice of a region with said fine grooves formed (wearing region), uneven wear of a tread that is further inside of the tire width direction from said region is inhibited.
At the time of driving, shoulder portions of a tread are subject to the force in lateral direction from a road surface as well as to the force in longitudinal direction. This tendency is particularly remarkable in a tire installed in a front wheel. In the tire of patent document 1, due to the lateral force from a road surface, the region sandwiched by sub-grooves (wearing region) falls inside of the tire width direction, sometimes causing uneven wear for failing to absorb friction energy caused by friction between the tire and the road surface.
In the tire of patent document 2 likewise, since the rigidity of the region with the three fine grooves formed (wearing region) is low, the region falls inside of the tire width direction thereby sometimes degrading the inhibition effect of uneven wear. Further, since provision of the three fine grooves is required, degree of design freedom of a tread pattern is degraded or since controlling said friction energy is difficult, there also lies a problem of crack generation at the groove bottoms of the outer two fine grooves.
Therefore, the object of the present invention is to provide a pneumatic tire capable of effectively inhibiting uneven wear of shoulder portions of a tread against the lateral force from a road surface as well.